A stent is commonly used as a tubular structure left inside the lumen of a duct to relieve an obstruction. Commonly, stents are inserted into the lumen in a non-expanded form and are then expanded autonomously (or with the aid of a second device) in situ. When used in coronary artery procedures for relieving stenosis, stents are placed percutaneously through the femoral artery. In this type of procedure, stents are delivered on a catheter and are either self-expanding or, in the majority of cases, expanded by a balloon. Self-expanding stents do not need a balloon to be deployed. Rather the stents are constructed using metals with spring-like or superelastic properties (i.e., Nitinol), which inherently exhibit constant radial support. Self-expanding stents are also often used in vessels close to the skin (i.e., carotid arteries) or vessels that can experience a lot of movement (i.e., popliteal artery). Due to a natural elastic recoil, self-expanding stents withstand pressure or shifting and maintain their shape.
As mentioned above, the typical method of expansion for balloon expanded stents occurs through the use of a catheter mounted angioplasty balloon, which is inflated within the stenosed vessel or body passageway, in order to shear and disrupt the obstructions associated with the wall components of the vessel and to obtain an enlarged lumen.
Balloon-expandable stents involve crimping the device onto an angioplasty balloon. The stent takes shape as the balloon is inflated and remains in place when the balloon and delivery system are deflated and removed.
In addition, balloon-expandable stents are available either pre-mounted or unmounted. A pre-mounted system has the stent already crimped on a balloon, while an unmounted system gives the physician the option as to what combination of devices (catheters and stents) to use. Accordingly, for these types of procedures, the stent is first introduced into the blood vessel on a balloon catheter. Then, the balloon is inflated causing the stent to expand and press against the vessel wall. After expanding the stent, the balloon is deflated and withdrawn from the vessel together with the catheter. Once the balloon is withdrawn, the stent stays in place permanently, holding the vessel open and improving the flow of blood.
In the absence of a stent, restenosis may occur as a result of elastic recoil of the stenotic lesion. This problem is not eliminated with a stent since the prolapse of plaque or tissue within the stent itself will occur in the unsupported areas called “cells”. All stents have cells and they generally fall into two groups: open-cell or closed-cell designs or architectures which facilitate plaque or tissue prolapse to varying degrees. Closed-cell refers to the fact that each opening or cell is isolated from any other by a full-connected metal perimeter. An open-cell design has openings or cells that can communicate with other cells around the circumference since they are not fully surrounded by a metal perimeter (there are gaps or “open” passages that lead to other cells). Although a number of stent designs have been reported, these designs have suffered from prolapse and a number of other limitations.
Additionally, the flexibility of the stent is important for maneuvering the stent through a vessel and less flexible stents are a drawback. The general construction or design of the stent, i.e. the open-cell or closed-cell design, is indicative of the ultimate flexibility or stiffness of the stent (open cell stents are more flexible). Another consideration is the cell size/shape as it pertains to accessing vessel side branches (“side branch access”). As is known in the field, side branch access is obtained by placing a balloon within one of the cells and expanding the structure or framework defining the cell with the balloon by inflating the balloon at a high pressure wherein the expanded cell is dilated to a larger size than the original expanded size of the cell. If the dilated cell size is too small, the less the ability to provide side branch access when the stent is expanded or deployed. If the cell deforms too much when ballooned, uniform coverage around the side branch will not be maintained and cause additional prolapse.
Accordingly, to date, there have not been any stent designs, that specifically address these drawbacks in an efficient and cost-effective manner. The invention described here is aimed at allowing small cell size with great flexibility without increasing prolapse.